1. Field of the Invention
This invention relates to a thermal print head for use in thermal printers and facsimile machines.
2. Prior Art
There is a tendency that a number of recently developed thermal print heads have their partially glazed glass with a heating element arranged toward an end face of a print head substrate, irrespective of the type of printer, serial or line, to improve the printing quality. This is because the arrangement that the head is inclined at a certain angle with respect to the sheet surface enables not only the ink ribbon peeling angle to be larger but also the head pressing force to be focused, thereby improving the printing quality. For this reason, the conventional thermal print head has been arranged as shown in FIGS. 1(a) and 1(b). That is, in FIG. 1(a), numeral reference 1 designates a thermally resistant insulating substrate, and numeral reference 2 designates a partially glazed glass made of glass material formed on the insulating substrate. A heating element 3 is arranged on the partially glazed glass 2. Further, a thin film common electrode 5 and an individual electrode 6 are formed on the heating element 3 so as to provide a predetermined space therebetween on a top of the glazed glass 2, and to make part B of the thin film common electrode 5 as small as possible. A protective film 7 is also formed on the heating element 3 through the thin film common electrode 5 and the individual electrode 6. A heating action as a thermal head is conducted through a heating element section 60 which is defined by the predetermined space between the thin film common electrode and the individual electrode. A thermal head as shown in FIG. 1(b) is fundamentally the same in construction as that of FIG. 1(a). However, in FIG. 1(b), a thick film common electrode 8 is arranged on the end and rear surface of the substrate 1 to ensure the adequate current capacity of the thin film common electrode 5 whose size is reduced.
However, since arrangement of the partially glazed glass 2 toward the end face of the substrate necessarily causes the heating element 3 to come closer to the end face of the substrate, the space for the thin film common electrode is reduced, thereby entailing the following problems.
(1) The current capacity of the thin film common electrode is decreased, so that when a number of dots are energized concurrently, a voltage drop occurs, thereby reducing the printing density.
(2) The utilization of the end face and rear surface as the common electrodes as shown in FIG. 1(b) will increase the cost of manufacturing the head.
(3) In the embodiments of FIGS. 1 (a) and 1 (b), at least a distance of 200 to 300 .mu.m is required between the edge of the head substrate and that of the partially glazed glass. This means that the more the number of dots is arranged on the head, the more it is difficult to near the glazed glass to the end face of the head.
In an attempt to overcome the above problems, a method has been disclosed in Japanese Patent Application No. 132580/1985.
The above patent application proposes a head whose structure is fundamentally the same as that of FIG. 1(b), as shown in FIG. 2. In FIG. 2, a thick film common electrode 4 is arranged under the partially glazed glass 2, that is, between the glazed glass 2 and the substrate 1. This is to increase the current capacity of the thick film common electrode 4, thereby contributing to appreciably improving the above problems (1) to (3). However, to provide larger angles of inclination of the head and of peeling of the ink ribbon, it is necessary to make the distance between the edge of the partially glazed glass 2 and that of the substrate 1, which is still 50 to 100 .mu.m, smaller.
To manufacture the conventional end face type thermal print head, a method shown in FIG. 3 has been known. In this method, one end of the substrate 1 is provided with a partially glazed glass 2; and the heating element 3 is arranged on the partially glazed glass 2; and the substrate 1 is cut.
However, such method of fabricating a thermal head by forming the partially glazed glass on the end face of the substrate in this way has a limit in that it can provide a maximum of only two lines of head a substrate for the serial printer and only two chips of head for the line printer, thereby raising the manufacturing cost.